This invention relates to the production of coarse, strong alumina hydrate particles from pregnant Bayer liquor. Specifically, the invention provides a precipitation system of several stages wherein each stage has a predetermined function and the combined and cooperative effects of these stages result in the production of coarse, strong alumina hydrate particles in high yield.
In the well-known Bayer process of alumina hydrate production, alumina-containing ores, such as bauxite, are digested with an alkaline solution. Digestion is carried out at high temperatures and pressures and it results in the generation of a supersaturated or "pregnant" liquor containing the alumina values of the ore. From this liquor the alumina values are usually recovered by precipitation. A detailed description of the conventional Bayer process can be found in "The Chemical Background of the Aluminium Industry" written by T. G. Pearson and published by The Royal Institute of Chemistry, G. Britain, as "Monograph No. 3, 1955."
It has long been the aim of Bayer process operators to precipitate as much alumina hydrate as possible from the pregnant liquor in order to maximize the efficiency of the process. It is also a goal in the Bayer process to obtain coarse, strong alumina hydrate particles in the precipitation step. If the hydrate particles are fine and weak, considerable problems will arise in the subsequent hydrate processing steps and in particular in the calcination stage where fineness results in significant dust generation and handling losses. Also, if the calcined fine alumina hydrate is used in the electrolytic production of aluminum, fineness can cause further processing difficulties, for example, in dry scrubbing of reduction cell offgases. Consequently, it has been a major consideration in the Bayer process particularly since the inception of these dry scrubbing processes to produce coarse, strong alumina hydrate particles in high yield.
In practice however these two objectives could not be achieved simultaneously. Either coarse alumina hydrate is produced at a reduced yield or in the alternative, a fine alumina hydrate is generated but in yields exceeding the ones obtained in the production of coarse alumina. Although both the coarse and the fine alumina hydrate can be calcined to the "sandy" or to the "floury" state, which expressions are well-known to Bayer process operators, in accepted plant operating practices coarse alumina is almost always calcined to the "sandy" state; while the fine alumina hydrate is generally calcined to the "floury" state to minimize product dusting and losses.
Based on the foregoing operating principles two distinct Bayer process operations evolved over the times.
The Bayer process based on the production of coarse alumina hydrate at lower yields is extensively practiced in the United States and is commonly referred to as the "American Bayer Process". In the American Bayer process, the coarse alumina hydrate is obtained by extracting the alumina values from bauxite with a caustic solution which has a sodium hydroxide concentration in the range of about 165 g/l to 190 g/l expressed as Na.sub.2 CO.sub.3 equivalents. The alumina (Al.sub.2 O.sub.3) content of the produced sodium aluminate solution is usually kept within the range of about 99 g/l and 135 g/l to obtain an alumina to caustic ratio (A/C) within the range of about 0.600 and 0.700. These concentration limits, coupled with the control of the temperature conditions and the seed quantity added to precipitate alumina hydrate, result in the production of a desirable sandy alumina hydrate product but at the cost of yield. Additionally, the "spent" liquor, that is, the liquor obtained when the alumina hydrate which precipitated is removed from the residual solution, is relatively dilute. Consequently, more energy is required to concentrate the spent liquor because more weight of liquor is being circulated with about the same loss of heat per unit of weight circulated. Also, more capital equipment is required due to the larger weight of liquor being circulated.
In Europe the energy-intensive nature of the Bayer process has long been recognized. Consequently, in Europe the Bayer process is practiced in a modified form which is generally referred to as "European Bayer Practice". In the European Bayer practice significantly higher caustic concentrations are used for the digestion of aluminous ores, generally in the range of about 220 g/l to 300 g/l. The spent liquor resulting from the precipitation step is more concentrated than the spent liquor of the American Bayer practice and thus requires less energy. Also, due to the use of the higher caustic levels in digestion, the quantity of alumina which can be dissolved by a unit volume of caustic is also higher. Further, the A/C ratios of European pregnant Bayer liquors is generally lower than those found in the American Bayer practice and are usually in the range of about 0.575 to 0.625. Although the European Bayer practice uses less energy per unit weight of alumina hydrate produced and also results in higher yields due to the higher alumina concentrations of the generated pregnant liquor, this process also has its definite disadvantages. These disadvantages are associated with the recycling of large quantities of alumina trihydrate seed and the production of a fine alumina hydrate during the precipitation step which then must be highly calcined to the "floury" state to make it less dusting. This fine, highly calcined alumina causes significant dusting problems because of its fineness and it cannot be utilized for the dry scrubbing of the offgases generated in the reduction process. Also, the fine floury alumina is not free flowing and therefore cannot be readily charged to reduction cells as feed.
Considering the advantages and disadvantages of both types of Bayer practices it becomes clear that it would be of great importance to provide a Bayer process which would result in a strong, coarse alumina hydrate in high yield and in addition achieve a reduced energy input per unit weight of hydrate produced. Since both the American and European Bayer processes have been practiced for many decades, it is only natural to assume that such a "combination" process has been developed. The published art in the Bayer process treatment of aluminous ores, however, indicates that most of the attempts were made to improve the individual processes rather than finding a process which would incorporate the advantageous features of both without the accompanying disadvantages.
In the following a representative sampling of the published "Bayer Process" patents is provided to indicate the state of the art with particular emphasis on precipitation techniques. The "American Bayer Process" and improvements thereto are shown in the following: U.S. Pat. Nos. 1,943,786 (Cowles), 2,653,858 (Brown), 2,707,669 (Houston et al), 3,486,850 (Day), 3,649,184 (Featherston) and 4,014,985 (Haleen et al) and British Pat. No. 1,045,216.
The European Bayer practice is represented by the following art: U.S. Pat. Nos. 3,480,388 (Michel), 3,545,923 (Mercier et al) and also by German Offenlegungsschrift 2,531,646 (Tielens et al--published Feb. 3, 1977), German Pat. No. 2,030,732 (Ferenc et al--granted Nov. 10, 1977) and German Offenlegungsschrift 2,623,482 (Schepers--published Dec. 8, 1977).
The instant invention provides a precipitation scheme which results in the production of the desired strong, coarse alumina hydrate at a significantly increased yield with a simultaneous reduction in the energy input required to produce the alumina hydrate. This is achieved by employing the high caustic concentration generally utilized in the European Bayer practice in combination with a three-stage precipitation system, wherein although each stage has a separate and distinct function, these stages are integrally associated. In the following a detailed description of the functions and operational characteristics of the novel system is provided.